document _has_feature(objc_default_synthesize_properties).

// rdar://11676972


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158665 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.html

@@ -1201,6 +1201,16 @@ tests the availability of array literals;
 dictionary literals; <code>__has_feature(objc_subscripting)</code> tests the
 availability of object subscripting.</p>
 
+<!-- ======================================================================= -->
+<h2 id="objc_default_synthesize_properties">Objective-C Autosynthesis of Properties</h2>
+<!-- ======================================================================= -->
+
+<p> Clang provides support for autosynthesis of declared properties. Using this
+feature, clang provides default synthesis of those properties not declared @dynamic
+and not having user provided backing getter and setter methods.
+<code>__has_feature(objc_default_synthesize_properties)</code> checks for availability
+of this feature in version of clang being used.</p>
+
 <!-- ======================================================================= -->
 <h2 id="overloading-in-c">Function Overloading in C</h2>
 <!-- ======================================================================= -->